In an internal combustion engine, generally, unburnt gas (referred to as blow-by gas) leaks through a gap between piston and cylinder during a compression stroke. When the blow-by gas (especially, NOx contained in the gas) is brought into contact with engine oil, it promotes degradation caused by oxidation. In view of this, there has been known a blow-by gas recirculation device for recirculating blow-by gas to an intake system of an engine for the purpose of treatment so as to reduce the risk of contact between the blow-by gas and engine oil.
In, for example, a blow-by gas recirculation device disclosed in PTL1, a communication hole is formed to allow a crankcase and a chamber inside of a cylinder head cover to communicate with each other, and then, the blow-by gas staying inside of the crankcase is led into the chamber inside of a cylinder head cover, to be thus recirculated to an intake passage. Additionally, a suction port and a discharge port at a vacuum pump for supplying a negative pressure to a brake booster are opened with respect to the crankcase and the chamber inside of a cylinder head cover via conduits, respectively, and furthermore, a control valve is disposed on the conduit connected to the crankcase in such a manner as to be opened only when a difference in pressure between the crankcase and the chamber inside of a cylinder head cover becomes a predetermined pressure value or higher. In this manner, during the recirculation of the blow-by gas in which the control valve is open, by the vacuum pump, the blow-by gas is sucked from the inside of the crankcase and is fed to the chamber inside of a cylinder head cover.